I'm not a parent, but this seems an awfully important issue to get right when it comes to children.
We also amend with family and community interaction as much as we can, as well as constantly engaging with them and teaching them that they can trust us in anything that is on their mind. Bringing up how we were raised seems to also aid (e.g. we show them kids shows that we grew up with, we mention that we didn't have cell phones as children, talk about our parents and grandparents times, etc.). I see some parents leaving their kids to watch whatever they want on youtube, and it's quite astonishing what garbage they watch. We pick and choose what shows and programs they're allowed to watch, again, mainly relying on shows we have already seen as children and which instill good values. Thankfully many of them can be found online. And modern incarnations of certain shows are treated as suspicious until proven innocent after what we're seeing them do.
Of course having peers of similar values is also very important, which is why school choice plays a big role, and public school is off the table (though I know not everyone can do the same, but it does not change the fact that it is a huge factor).
This is a much larger issue than just a phone. Awareness of the great damage that can be caused by social media of all forms is just the first step.